1. Field of the Invention
The present invention relates to a clamping system for machine tools, for the detachable, non-positive connection of two clamping system parts as a point of separation within a modularly constructed workpiece holding system or tool holding system, or as an interface between a workpiece support system or tool support system and a machine tool spindle.
2. Discussion of Related Art
The use of modern, regulated machine tools having drive systems for high machining speeds enables a significant increase in productivity through a reduction of machining times, due to an increase in cutting and advance speeds. In general, the drive technology for the main drives of machine tools meets the demands of high rotational speed, rotational rigidity, and dynamics made by high-speed machining. For example in cutting, new cutting materials such as oxide ceramics and polycrystalline diamond enable significantly higher cutting speeds, so that these machine tool components allow high-speed machining, with cutting speeds of, for example, 500 to 10,000 meters per minute in machining operations. However, the maximum machining speeds that can be achieved are determined by the forces that act on the tool or workpiece at these high cutting speeds, and by the safety aspects that must be taken into account in this context.
Thus, high demands are placed both on the machine tool spindle bearing system and on the workpiece support system or tool support system, as a weak point in the force transmission path from the machine tool to the tool or workpiece. In particular in clamping systems for rotating tools or workpieces, the highest degree of precision is required, because at the rotational speeds that are standard today of up to 30,000 RPM, very large forces occur, and even the slightest imbalances can destroy the sought precision in machining, or can fail to meet the required safety standards. Besides a high static rigidity, and a high dynamic rigidity in the case of rotating tool or workpiece systems, the demands made on a workpiece or tool clamping system include high-speed capacity and a high repeat accuracy given an automatic changing of tools or workpieces.
Clamping systems in the sense of the present invention are standardly distinguished in that one of the two clamping system parts to be connected has a cylindrical or conical hollow shaft, and the other part has a correspondingly formed receiving section in order to receive the hollow shaft with a precise fit. A clamping device situated concentrically in the receiving section is used for the clamping of the two clamping system parts that are to be connected.
Thus, for example, from DE 4220873 A1 a modular clamping system is known having a clamping device in the form of a system of clamps, via which the hollow shaft of the one clamping system part is drawn into the receiving section of the other clamping system part, producing a high surface pressure between the joining surfaces of the hollow shaft and of the receiving section, and also between end surfaces of the two clamping system parts that are to be coupled. The system of clamps has in particular a clamping element that can be attached in the receiving section, which, in the joined state of the clamping system parts, extends into the hollow shaft up to a point near an inner shoulder, and has a radial recess in which two clamping elements are received. Using an operating element, the clamping elements can be forcibly moved, in opposite directions, into and out of engagement with an undercut clamping shoulder of the hollow shaft through a radial opening in the hollow shaft and in the receiving section.
Another known clamping system, as described for example in the document DE 3807140 C2, includes a clamping device having a plurality of clamping claws that are distributed uniformly over the periphery thereof, and are situated loosely, or at least are not uniquely fixed, these claws being attached in positively locking fashion in the receiving section, and being brought to rest on an undercut clamping shoulder formed in the hollow shaft.
In addition, DE 19753663 A1 discloses a clamping system having a clamping device in the form of a clamping tongue or clamping fork, which, in the joined state of the two parts, extends into the hollow shaft of the one part, and has at least two clamping elements that can be moved in opposite directions, as well as an actuating device that drives the clamping elements. Here, the clamping elements can be brought into and out of engagement with an undercut clamping shoulder of the hollow shaft. The clamping elements are realized as head segments of oblong clamping elements situated essentially parallel to the longitudinal axis, or axis of rotation, of the receiving section, the foot segments thereof being connected with one another.
Recently, the hollow shaft cone clamping system has proven successful because it offers the advantage that, in the joined state of the two clamping system parts, depending on the embodiment the clamping elements used for the clamping, i.e., the clamping bodies or clamping claws, ensure not only that a sufficient surface axial pressure is produced between the end surfaces of the two clamping system parts to be connected, but also that the hollow shaft experiences a certain radial expansion. In this way, the precision of fit between the hollow shaft and the receiving section, as well as the radial positioning accuracy, i.e., the axial alignment, of the two clamping system parts is improved. Precision of fit and accuracy of positioning are the characteristics which a clamping system must have with respect to stability, i.e., static and dynamic rigidity, at high machining speeds in particular.
However, it has turned out that in particular given the constantly increasing rotational speeds, a stable and axially aligned clamping of the two clamping system parts that are to be connected becomes increasingly difficult. The cause for this is the fact that the very high rotational speeds cause the centrifugal forces to increase considerably, so that the clamping forces, which provide the fixing of the two clamping system parts and the transmission of torque, decrease. On the one hand, an increase of the clamping forces is possible only up to predetermined load limits, and on the other hand, as the kinetic energy stored in the clamping system increases the safety risks also increase. Thus, the joining surfaces of the receiving section and of the hollow shaft, and also the end surfaces and the clamping surfaces in the joined state of the two clamping system parts, must be manufactured with greater precision in particular for rotating tools, in order to achieve a seating that is as precise as possible of the hollow shaft in the receiving section, and thereby a stable clamping.
Despite numerous measures aimed at meeting these increasing demands (for example, improved materials have been used, devices have been proposed for monitoring the machining and for correcting the tool settings, etc.), it has nonetheless been observed that, in particular for the case of automatic tooling systems, the high demands placed on a clamping system are not met in such a way as to achieve the sought positioning accuracy of the two clamping system parts to be connected, and the static and dynamic rigidities required at high machining speeds.